- change the images to be upside down until presentation to increase compatibility with shaders designed for OpenGL

- clamp scissors fully to avoid NVidia's awful drivers locking up the entire system if they end up out of bounds
- perform buffer clears as part of the render pass. this puts some restrictions on how FRenderState.Clear can be used
- add an offset uniform to the present shaders so the vulkan target can flip the image during presentation

src/rendering/gl/renderer/gl_stereo3d.cpp

@@ -173,6 +173,7 @@ void FGLRenderer::prepareInterleavedPresent(FPresentShaderBase& shader)
 		screen->mScreenViewport.width / (float)mBuffers->GetWidth(),
 		screen->mScreenViewport.height / (float)mBuffers->GetHeight()
 	};
+	shader.Uniforms->Offset = { 0.0f, 0.0f };
 	shader.Uniforms.Set();
 }